1. Field of the Invention
This invention relates to a recording apparatus for recording desired information signals on a magneto-optical recording medium by applying a magnetic field and simultaneously radiating a light beam to the magneto-optical recording medium, using a semiconductor laser device as a light source radiating the light beam.
2. Description of the Related Art
As a recording medium for recording the information signals, a magneto-optical disc has hitherto been employed, in which the magneto-optical effects are utilized for recording the information signals. The information signal recording system, employing such a magneto-optical disc as the recording medium, is the magnetic field modulation system in which the direction of the magnetic field to be applied to the disc is modulated in accordance with the information signals desired to be recorded on the disc.
With a information signal recording system employing the magnetic field modulation system, a light beam emanated from a semiconductor laser is continuously radiated on the magneto-optical disc for heating a magnetic film area of the magneto-optical disc in which to record the information signals in order to decrease the coercivity of the magnetic film area in which to record the signals. An external magnetic field, having the direction of the magnetic field modulated in accordance with the information signals desired to be recorded, is applied to the magnetic film area of the magneto-optical disc decreased in coercivity with the aid of an electro-magnet. The direction of the magnetic film of the magneto-optical disc area with decreased coercivity is changed for recording the desired information signals.
For reproducing the information signals, thus recorded on the magneto-optical disc as changes in the direction of magnetization, the Kerr effect or the Faraday effect, relating to the interaction between the light and the magnetism, is utilized. When a light beam is incident on a magnetized member in a direction parallel to its direction of magnetization, the plane of polarization of the reflected light and that of the transmitted light are rotated by angles of rotation which are the Kerr rotation angle and the Faraday angle of rotation, respectively. If the direction of the magnetic domains differs with respect to the light incident direction, a difference in the direction of rotation is produced. The difference in the direction of the Kerr rotation angle of the reflected light is read for reproducing the information signals.
For such reproduction, there is induced a phenomenon in which the semiconductor laser undergoes self oscillation to increase the output light by the reflected light from the disc surface being fed back to the semiconductor laser. An optical pickup device of the type in which such phenomenon is positively utilized so that information signals on the magneto-optical disc are detected from changes in an output light of the semiconductor laser caused by the reflected feedback light is called the self-coupled optical pickup (SCOOP).
With the SCOOP, a smaller number of component parts suffices, while the device may be reduced in size, weight and cost. However, it suffers from noise generated in the semiconductor laser by the reflected light being fed back to the semiconductor laser which is the light source. This noise is termed hereinafter as the SCOOP noise. Of course, such SCOOP noise is produced in an optical pickup device other than the SCOOP, that is an optical pickup device not utilizing the phenomenon of self oscillation of the semiconductor laser device by the reflected light.
The SCOOP noise is produced by the fact that the semiconductor laser oscillated in a single longitudinal mode by dc current driving is oscillated in a multiple longitudinal mode due to the feedback reflected light from the magneto-optical disc accompanied by minute displacement. Such single longitudinal mode oscillation and multiple longitudinal mode oscillation are induced alternately depending on the state of displacement of the magneto-optical disc. A larger laser output light and a smaller laser output light are generated during the single longitudinal mode oscillation and the multiple longitudinal mode oscillation, respectively.
The SCOOP noise is generated by the semiconductor laser being oscillated in the single longitudinal mode or in the multiple longitudinal mode by the reflected feedback from the magneto-optical disc to deteriorate reproduction of the information signals.
Thus it has been contemplated to superpose the high frequency superposition current shown in FIG. 11 on the driving current for the semiconductor laser during reproduction, as indicated in FIG. 11. The high frequency superposition current has completely ON periods and completely OFF periods, as shown in FIG. 13. Consequently, should such high frequency superposition current superposed on the driving current be used for driving the semiconductor laser, the semiconductor laser is oscillated in a multiple longitudinal mode by on/off control. The light output of the semiconductor laser oscillated in a multiple longitudinal mode is smoothly changed against displacement of the magneto-optical disc to suppress the SCOOP noise.
On the other hand, the SCOOP noise has hitherto not been taken into account when recording information signals by magnetic field modulation as discussed above. The reason is that the laser light output radiated during recording on the magneto-optical disc from the semiconductor laser is of a high power and hence was not thought to be affected significantly by the SCOOP noise.
The system of recording information signals using the magnetic field modulation system is explained by referring to FIG. 1.
In this figure, the laser light radiated by a semiconductor laser device 51, having its oscillation power controlled by automatic power control (APC) by an automatic power control (APC) circuit 52, is detected by a photodiode 54 constituting an I-V converter 53 along with a resistor 55. The photodiode 54 is arranged facing the front side of the semiconductor laser device 51, and detects part of the laser light beam split by an optical system, such as a beam splitter.
Meanwhile, when recording information signals on the magneto-optical disc by magnetic field modulation, the SCOOP noise slightly changes a light output of the recording semiconductor laser device. Such change in the laser light output will produce fluctuations in the positions of the modulated perpendicular magnetic field. These fluctuated modulated magnetic field positions represent the optical jitter.
The generation of such jitter is explained by referring to FIGS. 1 and 2.
It is now assumed that the semiconductor laser device 51 is driven by the APC circuit 52, and the laser light L outputted from the semiconductor laser device 51 is detected by the photodiode 54 of the I-V converter 53, as shown in FIG. 1. If the SCOOP noise is produced by the semiconductor laser device 51, random power fluctuations, that is jitter, continuing for 1 to 10 .mu.sec and occasionally for 20 .mu.sec, are generated, as shown in FIG. 2.
Such jitter during recording leads to error rate deterioration during reproduction which is ascribable to recording-related factors. For reproducing digitally recorded information signals, it is necessary to extract PLL clocks. However, if the jitter is produced, the PLL cocks tend to follow the jitter, so that errors on the time axis, that is timing errors and hence PLL clock extraction errors are produced.